What you're talking about is not huge risk for a hobby project.. a few things to think about.
It's not really the \$V_F\$ at the operating current that matters, it's the \$V_F\$ at the maximum 'off' brightness that is acceptable. That voltage might be as low as 1.4-1.5V volt for a low-current red LED- in a dark room they can be quite visible with microamperes of current. Driving the output to the 3.3V (nominal level) gets us 3.3V on the output. A fresh alkaline cell with minimal load might be 1.63V/cell at room temperature (just measured one), so 3 would be 4.89V. That leaves you with 1.59V across the LED + resistor (nominal, not allowing that the 3.3V might be a few percent low).
That's way too much to ensure it's not emitting a whole bunch of light.
So, we tri-state it- that allows the output to go maybe a bit above Vcc without much current flowing. 300mV is safe, the datasheet says 500mV absolute maximum. At 500mV, we'd have 1.09V across the LED, probably enough to ensure it's off, at least under nominal conditions. The 'absolute maximum' figure is never a good thing to design to, but usually there's a caveat on this particular figure that allows that voltage or a bit more if the current is limited.
So, I do think this will work (with tri-state, not push-pull), and I also think it's acceptable enough for a hobby project assuming nobody is going to be using a battery eliminator on the circuit in the future*. Keep in mind the margin on the red LEDs is minimal and consider eschewing red in favor of yellow or orange. Or, simply add a silicon diode in series with the red LEDs (one diode can be used for several LEDs).
- If the ESD protection network in the ATMEGA328P does begin to conduct it will tend to raise up the 3.3V supply, out of control of the regulator. This is not a good thing for stability and could conceivably damage something, though the ATMEGA328P-M itself is rated for 5V operation.
I once did something like this in a commercial product (to drive a series string of LEDs with high Vf using a 5V constant-current output), but I designed a power supply with just the correct oddball voltage and appropriate temperature coefficient to match the LEDs and thus optimize the situation. I think the supply was around 8-9V. Worked a treat, easily from -20°C to 80°C (spec was 0-50°C).
Is this possible,
Yes
is the DMX to Meanwell architecture the way to go?
Use multiple Mean Well HLG-240-48A or HLG-240-48B (cheapest cost per watt)
The type A has a pot to vary the current between 50% and 100%.
Type B uses 0-10v, or a resistance to set the current.
Use strings of 16 LEDs (white, blue, green) or 21 red in series (≈45v Vf). Connect as many of the strings in parallel as each HLG can support. Try to match forward voltages of the strings when connecting in parallel.
The parallel wiring will cause some current imbalance between strings. If the forward voltages are not matched well enough you will need to add current balance circuits. The easy way is to use an LM3466 with each string. Keep the temperature down with good thermal management.
Best Answer
The problem is most likely that the mosfet does not have enough voltage to fully turn on. Vgs is probably not high enough. I couldn't find the datasheet for the CMPD7002 (or any part named that). First make sure the Vgs will give you enough current, find how much Vgs you need for the Rds on. You'll need to do one or more of these things:
1) If the GPIO's on the microcontroller are 3.3V tolerant (or if 3.3V is fine with a pull down) Then use open drain on the GPIO and a pull up to 3.3V
2) Use a circuit or buffer to translate 1.8 to 3.3V
3) Select a different mosfet that turns on at 1.8V and has a low rdson at 1.8V.
4) Calculate the current vs LED drop and size R1 correctly.